Adventures in Agroecology and Food Systems

Vermicompost at Jenner Farm – Guest Blogger Eleanore Nelson

Congratulations to Eleanore Nelson, whose senior project was picked for the Environmental Studies baccalaureate presentation at Prescott College. Here’s Eleanore’s description of her project. It was a pleasure to be your mentor Eleanore!

-Allison Jack

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In current industrial large-scale agriculture, the “cycle” of the nutrient cycle is completely fractured. Currently, food is grown in soil, harvested, processed, packaged, and consumed by people or animals maybe in another state or country. The excrement that comes out of both humans and animals are jammed packed with vital nutrients that are actually limiting to a plant’s growth. Sadly, this fecal matter isn’t returned to the soil from which the fresh food was originally grown. It is flushed down the toilet to be treated or in the case of animal manure, kept in large man-made lakes, where anaerobic decomposition takes over and methane gas (CH4) is emitted. Livestock enteric (intestinal) fermentation and manure management in 2010 accounted for 21.2% and 7.8% of the total CH4 emissions in the US respectively (US EPA 2012).

Prescott College is in the process of developing animal traction for our school’s farm, located in Skull Valley, AZ. The addition of draft animals will create independence from the fossil fuels that power our tractor. However, with the independence from fossil fuels requires additional planning: developing a perennial pasture, building a structure for the draft animals to live under during the cold winter months (yes it does get cold in Arizona!), and properly recycling their manure.

For my senior project I designed and constructed a pilot scale model of a vermicomposting system that could be used to properly compost horse manure to yield a very valuable product for crop production.

So why vermicompost and not just regular compost?

By feeding animal manures, food scraps and other organic matter to worms, you are making the compost product richer than just thermophilic compost (compost that has experienced temperatures over 149F). First of all there is a very diverse soil microbe community present in the vermicompost. When the worms consume organic matter, their guts cover their castings (the technical term for worm poop) with a mucous that attracts other soil organisms like mites or nematodes, which in turn attract others. Soon enough, the vermicompost is teeming with a diverse collective of decomposers. Additionally, studies have shown that there is an increase in plant available nutrients in vermicompost compared to compost, that vermicompost suppresses plant pathogens in the soil, and, when used to recycle animal manures, a decrease in methane emissions.

Here’s my in-ground design that I like to call the “Vermi-pit”:

The vermi-pit is lined with concrete blocks. There is an insulated cover that goes on top of the system to help keep the vermicompost cool in the summer and warm in the winter. Additionally, there is a screen that divides the system.

Here’s myself (and along with other students) building it:

First, Matt helped me use the tractor to dig a pit into the ground.

Then we lined the bottom of the pit with concrete blocks in order to keep the gophers out of the vermi-pit. There’s about a 10 inch space in the middle where the 4’x 1’8’’ screen is placed where we laid down chicken wire to enforce gopher protection. We then lined the walls of the pit with concrete blocks (4’’ x 16’’ x 8’’) only two high, three for the width and four for the length, totaling 28 blocks. The screen was placed in the middle.

Then we backfilled in the space between the wall of the pit and the concrete blocks with soil.

Facing West Cottonwood Lane, the left side of the pit is about 14 ft3 (0.4 m3) and the right side is about 12 ft3 (0.34 m3).

I cut an insolated door in half, each piece acting as a cover for each side of the system.

Final Result

How it works:

The idea behind this design is the ability to conduct thermophilic composting on one side of the system, while the other side is being fed to worms. The thermophilic composting side will act as the battery to keep the vermicompost on the other side insulated and warm. Once the worms are done consuming the horse manure and food scraps on one side, they will migrate through the screen to compost that had just been under thermophilic conditions. Then one can harvest vermicompost without having to pick out worms for hours on end. This is called passive harvesting. Once the vermicompost is collected out of the pit, then horse manure, food scraps and agricultural residues can thrown into the pit to go under thermophilic composting, again acting as the battery to keep the other side, which is now being eaten by worms, warm and insulated.

The materials were all very relatively cheap. The most expensive purchase was for the worms. I acquired 10 lbs of worms, 5 lbs from a seller who fed his worms horse manure, and 5 lbs from a seller who fed her worms cow manure. I began by inoculating about 14-ft3 of aged horse manure with about 5-lbs of worms. I didn’t want to put all of my worms in there just incase something were to happen and all of them died, I was on a tight budget!

The insulated door that I placed on top of the system helped keep the vermicompost temperature at around 68 F, which is ideal for worms. Without the insulated cover and no worms, the temperature of the pile was ~50 F. Worms are very susceptible to cold and hot temperatures, and in Arizona we have both. During the summer in Skull Valley, the temperatures can reach above 100 F, and during the winter the temperatures can go below freezing. The door also helps in decreasing the rate of evapo-transpiration. Worms like very moist conditions (80-90% moisture content), which can be a roadblock in raising worms in such an arid climate. In the summer I plan on having a shade cloth over the entire system and removing the door so it doesn’t get too hot in the compost pile, and it has shade to prevent extreme water loss.

Although the climate in Skull Valley is very arid and can be both very hot and cold, the vermicompost is completely insulated in the ground. I observed temperatures between 66-68F for four weeks after inoculation. However, the right side, which I filled with field culls never reached a thermophilic compositing stage. So the battery idea may not be feasible. However, I did find mating worms and cocoons in the vermicompost, indicating that the temperature and moisture content are ideal. So worms can thrive in Arizona and do their work as master decomposers to add value to an already rich, all natural product. Additionally, since the worms are reproducing, the farm managers could harvest the excess worms and utilize them as a source of protein for their flock of chickens. Everyone wins when you reconnect the cycle!

Great – look forward to meeting you. I am on the Board of the Highfields Center for Composting here in Hardwick, which is working on a commercial scale vermicomposting project, using heat recovery from compost to keep the worm room warm. Check out http://www.highfieldscomposting.org.

This is outstanding. I am a hobbyist so lack any credentials at all. If it is not too bold I would like to ask a few of questions. I have been searching for the answer to the last question below (do you fill the bin with just manure “pellets” (meaning do you separate them out from the rest of the bedding and feed waste from the stable before putting them into the bin)]

1. How often, if at all, did you water the vermi bin?

2. It appears that aerobic composting requires a critical mass – pile size of at least 4 x 4 x 4 to generate the heat process, plus air. It seems that there might not have been enough of either, along with water, to get this side of things going

3. I have a thought (below) on using your fabulous idea but would like to ask a couple more on this first

Did you have to separate out the horse manure “Pellets” ( I love that phrase” from the bedding when you put it into the bins, or did you just fill the vermi side with complete stable waste?

Here is my thought, and any critcsism from you or others would be great

I am handling the landscaping – composting at a ten or so horse stable using windrows (6′ x 6′ x X’). Based upon your idea, I wonder if I can use the heat from the aerobic process if I separate the windrows enough so that once two adjacent rows are through the thermophilic phase I make a new row between them and seed (if needed) the two old rows with worms? In other words, I make a horizontal “new manure” sandwhich using “old manure” bread

1) So far we have only watered it once and it was completed in November.
2) We did not get thermophilic temperatures, but we weren’t sure if that was because the manure was well aged, or because the thermal mass was too small.
3) We got the horse manure from a neighbor, and it looked less like stable waste and more like it was shoveled out of a feeding pen. It appeared to be almost 100% manure.